The present invention relates to a control circuit for a semiconductor device having an overheating protection function for preventing breaking, due to overheating, of a power semiconductor element that is used for driving a vehicular lamp, for example.
A control circuit for a semiconductor device having an overheating protection function is known which includes a semiconductor device having an overheating protection function, an output state detecting means, and a control means. The semiconductor device having an overheating protection function is such that a semiconductor element such as a MOSFET, a temperature detecting circuit, a latch circuit, and a gate interrupting circuit are incorporated in one chip. The temperature detecting circuit detects temperature increase of the chip and supplies a detection output to the latch circuit. The latch circuit latches the detection output of the temperature detecting circuit. Controlled by the output of the latch circuit, the gate interrupting circuit interrupts a gate input to the semiconductor element. The output state detecting means detects the output state of the semiconductor device having an overheating protection function.
Being a CPU (central processing unit) or the like, the control means supplies a PWM (pulse width modulation) control signal to the semiconductor element and monitors the detection output of the output state detecting means regularly with monitoring timing that is each rise time point of the PWM control signal plus a prescribed time. The control means compares the logic value (1 or 0) of the detection output of the output state detecting means and that of the PWM control signal, and performs a control so as to stop supplying the PWM control signal to the semiconductor element if a state that the logic value of the detection output of the output state detecting means is different from that of the PWM control signal occurs consecutively a predetermined number of times or over a prescribed time (refer to Patent document 1, for example).
[Patent document 1] JP-B-3,585,105 (claim 1, paragraphs [0002]-[0012] and [0024]-[0033], FIG. 1)
In the above conventional control circuit for a semiconductor device having an overheating protection function, the output terminal voltage of the semiconductor element is supplied to the control means through an interconnection that extends from the semiconductor device having an overheating protection function to the dedicated control means for monitoring the semiconductor device. And an overheating/cutoff state of the semiconductor device having an overheating protection function is detected by measuring the output terminal voltage of the semiconductor element with the control means. This results in problems that the circuit is complex and large in size and hence the control circuit for the semiconductor device having an overheating protection function is expensive.
In the above conventional control circuit for the semiconductor device having an overheating protection function, to judge whether to stop supplying the PWM control signal to the semiconductor element, supply of the PWM control signal and a protecting operation are performed several times repeatedly. This results in a problem that deterioration due to overheating of the semiconductor element progresses though slightly.
Furthermore, in the above conventional control circuit for the semiconductor device having an overheating protection function, a control may be performed in such a manner that the supply of the PWM control signal to the semiconductor element is stopped because the control means operates erroneously due to disturbance noise though the semiconductor element is not overheated and cut off. There is a problem that this no load current comes to flow through a load and, if the load is a lamp, the lamp is turned off.